This application is based on application No. Hei 11-83740 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to a drive device, and more particularly, to a drive device using an electromechanical conversion element such as a piezoelectric element. The present invention also pertains to an apparatus equipped with a drive device, such as an X-Y drive table, a camera lens or a scanning tunnel electron microscope.
2. Description of the Related Art
The applicant has previously proposed a drive device that achieves a high, sub-micron level resolution using a piezoelectric linear actuator instead of a stepping motor.
For example, the drive device 1 shown in the perspective view of FIG. 1A was disclosed in Japanese Laid-Open Patent Publication No. Hei 10-225149 (10225149), wherein a drive unit 20 is grasped in the space 9 between a pair of fixed friction members 4 and 6 fixed on the top surface 2s of a base 2 such that the drive unit 20 remains in place via friction. One fixed friction member 6 is supported on the base 2 via support springs 8 and is pushed toward the other fixed friction member 4.
To explain in more detail, the drive unit 20 comprises a moving unit 24 that is connected to an object to be driven such as a lens or stage, which is not shown in the drawing, and that has a relatively large mass; a piezoelectric element 22; and a drive friction member 26 that has a relatively small mass. One end of the length of the piezoelectric element 22, along which length expansion or contraction occurs, is fixed to the moving unit 24, while the other end is fixed to one end of the shaft of the drive friction member 26. The drive friction member 26 is supported by the moving unit 24 such that it can freely move in the same directions in which piezoelectric element 22 expands or contracts.
To explain in further detail, the piezoelectric element 22 is a kind of electromechanical conversion element. It comprises multiple piezoelectric plates that undergo changes in volume when a voltage is applied to them, and expands or contracts along its length as the applied voltage changes. The moving unit 24 has an essentially column-like configuration, and has on its top surface screw holes 24a and 24b to mount an object to be driven which is not shown in the drawing. Parts of the body 24c of the moving unit 24 are cut out from the top such that two concave areas 24s and 24t are formed, in which are placed the piezoelectric element 22 and the drive friction member 26, respectively. The drive friction member 26 comprises a cylindrical main unit 26s and shafts 26t sticking out from either end of the main unit. Each shaft 26t is supported by the moving unit 24, such that the drive friction member 26 may freely move in the directions in which they extend. One shaft 26t is fixed to the piezoelectric element 22. The width of the main unit 26s of the drive friction member 26 is larger than that of the piezoelectric element 22 or the moving unit 24. As shown in FIG. 1B, the outer circumference of the main unit 26s is in contact with the surfaces 4s and 6s of the fixed friction members 4 and 6 that face the drive unit 20.
A drive pulse generating circuit not shown in the drawing is connected to each electrode of the piezoelectric element 22, so that a pulse voltage having sawtooth-shaped waves or full-wave rectified waves may be applied to the piezoelectric element 22. For example, the drive friction member 26, and therefore the moving unit 24 and the object to be driven, are moved along the fixed friction members 4 and 6, by causing the piezoelectric element 22 to expand and contract at different speeds so that sliding will occur between the drive friction member 26 and the fixed friction members 4 and 6 in opposite directions and with different magnitudes, or so that sliding will occur only when the piezoelectric element 22 expands or contracts.
In this drive device 1, when the position of the drive unit 20 changes relative to that of the fixed friction members 4 and 6, the pressure that the drive friction member 26 receives from the fixed friction members 4 and 6 changes depending on the elastic deformation and the change in the tilting of the fixed friction member 6, resulting in a change in friction between the drive friction member 26 and the fixed friction members 4 and 6, and therefore in a change in the driving conditions. Consequently, the driving speed and drive force change depending on the position of the drive friction member 26, relative to that of the fixed friction members 4 and 6.
The applicant therefore proposed an improved drive device 10 in which the fixed friction members 14 comprise a number of spring elements 15 mounted on the surface 12s of the base 12, and in which an essentially constant level of friction is applied between the fixed friction members 14 and the drive friction member 26 based on a construction in which only the spring elements 15 that are in contact with the drive friction member 26 warp. For example, multiple individual contact pieces 15 are formed by means of a number of spring pieces or fixed friction members 14 in which notches 14s are formed, as shown in FIG. 2.
Further, the applicant also proposed an improvement in which metal sheets, for example, are placed on the surfaces that come into contact with the drive friction member of the spring elements comprising the fixed friction members, so that the drive friction member does not become stuck between the spaces of the spring elements and the drive friction member smoothly slides against the metal sheets.
However, if the fixed friction members comprise a number of spring elements, in order to maintain the speed and thrust of the moving unit at an accurate level, accuracy in the processing and assembling of the fixed friction members must be increased. In addition, in order to adjust the friction, it is necessary to change the amount of warp by changing the degree to which each spring element of the fixed friction members encroaches in the movement path of the drive friction member. Further, because the fixed friction members are formed with a number of spring elements, a large number of components is needed.
The object of the invention is to provide a drive device that can reduce the fluctuation in friction between the fixed friction member and the drive friction member so that it is constant, using a simple construction.
In order to attain the object stated above, one aspect of the present invention is a drive device comprising an electromechanical conversion element that is connected to a drive pulse generator and expands or contracts, a moving unit that is fixed to one end of the length of the electromechanical conversion element, along which length expansion or contraction occurs, a drive friction member that is connected to the other end of said length of the electromechanical conversion element, and two elongated fixed friction members that extend and are fixed along the movement path of the drive friction member and that grasp the drive friction member in between them by means of friction. When the drive friction member is driven by causing the electromechanical conversion element to expand and contract by means of the drive pulse generator, the moving unit moves in a prescribed direction. Further, the drive device has a spacer that has side surfaces that are distanced from each other over a distance slightly smaller than the width of the drive friction member and that extend along the movement path of the drive friction member. The two fixed friction members are mounted to respective opposite side surfaces of the spacer. At least one of them includes a plate member having a spring capability such that an essentially constant level of friction is provided between the fixed friction members and the drive friction member along the movement path of the moving unit.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings, which illustrate specific embodiments of the invention.